The present invention relates to printing and graphic arts generally and more particularly to an apparatus for page composition and printing and a method for prediction and elimination of image content dependent artifacts.
The following terms, employed herein, are intended to have the meanings specified herein below:
Color Image
The term is intended to include image-comprising gradations of a single tone, such as black and white images.
Analog Representation of a Color Image
Any representation of a color image that resembles the original color image. The representation may appear upon a printed page, a proof or any other suitable substrate.
Digital Representation of a Color Image
Any representation of a color image which is expressed in discrete symbols, such as a computer file.
Color Characteristics of a Color Image
The characteristics of the color image, defined by individual elements of a representation of a color image, which directly represent a color or a component of a color.
Spatial Characteristics of a Color Image
Characteristics defining the arrangement of and the relationship between elements of a digital representation of a color image, such as pixels, which characteristics do not directly represent a color or a component of a color, Spatial characteristics include but arc not limited to resolution and format characteristics such as pixel by pixel encoding.
Ghost
Ghost is defined as the repeat of a previously printed image, one form roll revolution later.
Ghosting Prevention Bar
Ghosting prevention bar or ink collection bar is typically a rectangular area filled with color and placed on a signature/plate to prevent ghosting.
Plate, Signature, Imposition
Plate, Signature, Imposition have the same meaning as typically associated with that of a complete offset plate pattern that includes separate page images, registration marks, color control bars, ink collection bars and water trapping prevention bars.
Printer Spread
Imposed pages that have to keep the relative orientation between them for the printed product and pertain to the same printing form/plate.
The production of a book or magazine involves a large number of processes. Those processes may be grouped into five general categories: prepress, plate preparation, press set-up or xe2x80x9cmake readyxe2x80x9d, press control operations and postpress operations.
In prepress, the principal processes are graphics, image and text editing and composition. In recent years, this area has undergone revolutionary changes through the application of computer technology. State of the art systems are available for graphics, image editing and composition and for the specific functions of scanning, image processing and film and plate setting, such as the systems manufactured and sold by Scitex Corporation Ltd. of Herzlia, Israel, under the trademarks EverSmart Scanner, Dolov and Lotem. Plate preparation involves a technique known as imposition or signature assembly, which refers to the arrangement of pages on a film in a Computer-to-Film system, on a plate in a Computer-to-Plate system or on a plate in a Computer-to-Press system. The present state of the art in signature assembly is the use of computerized systems such as the system sold under the name Brisqueimpose by Scitex Corporation Ltd. of Israel. If the imposition is performed to film, the film is later used for contact exposure of printing plates. In the ease where the imposition is performed to plate, the stage of contact exposure of printing plates is eliminated. Press set-up or xe2x80x9cmake-readyxe2x80x9d takes place after preparation of the printing plate, and typically involves the steps of plate mounting ink key setting, which determines the supply of ink flow for that plate, solution or water setting, and other steps. If the imposition is performed on a plate in a Computer-to-Press system, plate mounting is not required. The ink key setting data is derived according to the image-on-plate coverage information, which is generated in the stage of computerized signature assembly. The ink key setting data may be generated by computerized systems such as the system sold under the InkPro trade name by Scitex Corporation Ltd. of Herzlie, Israel. Press set-up information, derived by the prepress systems, is typically communicated to the press using a standard Print Production Format communication protocol called CIP3, which is described at the Internet site http;//www.cip3.org/documents/technical_info/index.html. The ink key setting data generated by these computerized systems does not take into account, however, the wet ink trapping and accordingly does not provide correct ink key settings pen color.
U.S. Pat. No. 5,875,288 to Bronstein et al. discloses an integrated computerized system for use in color printing, having at least one digital representation of a color characteristic of at least one page to be printed and a digital storage memory for storing that digital representation. An imposition apparatus is connected to the storage memory, to receive the digital representation and arrange the digital representation in accordance with a desired plate layout, thereby to define a plate image. A press setup device extracts from the plate image the color characteristic and provides ink flow set-up data to a printing press in accordance with the extracted color characteristic.
A state of the art printing press is described in U.S. Pat. No. 4,936,211 to Pensavecchia et al. The patent discloses a printing apparatus which is intended to achieve complete computer control over the entire printing process, including plate generation, ink regulation and the start-up, print, hold, shut-down and clean up stages of the actual printing operation. Pensavecchia et al. also refer to a press including a workstation, which allows an operator to input a digital representation of an original picture to be printed. The workstation may include a CRT display and internal memory for storing image data, so that the impression to be printed may he previewed before panting. A keyboard is supplied, through which the operator may key-in instructions regarding the particular press run, such as the number of copies to be printed or the number of colors in the printed copies. The workstation is also intended to allow complete control over the operating modes of the press, including printing plate imaging, press startup procedure, ink flow regulation, dampening, print, pause, shutdown and clean-up sequences.
Despite the existence of sophisticated computerized prepress systems and printing presses, the press operator is often required to intervene in the printing process to maintain the target print quality. The deviations from the desired target print quality can be significantly reduced by taking into account the characteristics of the image to be printed. Typical examples of printing defects are ghosting, improper ink transfer, wet ink trapping, solution or water balance end others. These printing defects are typically discovered only during the printing process and in extreme cases may require complete press stop, re-imposition of the signature and the production of new plates, resulting in a significant loss of time and money.
Numerous efforts have been made to eliminate these image-dependent problems, in particular mechanical ghosting, which is the occurrence of unwanted patterns of higher or lower density created by the job layout, combined with the press""s inking ability. Mechanical ghosts can be seen in heavy solids or in heavy process work on both single and multicolor presses.
There are two types of mechanical ghosts, sometimes referred to as xe2x80x98positivexe2x80x99 and xe2x80x98negativexe2x80x99. In xe2x80x98negativexe2x80x99 ghosts, illustrated in FIG. 1A, spot 20 of a slightly lighter tone (a ghost) will appear in a flat solid 22. It takes on the form of another solid 24 on the plate 26. 250-300 mm (10-12 inches) ahead of the ghost. The actual distance depends on the particular press construction. xe2x80x98Positivexe2x80x99 ghosts, illustrated in FIG. 1B, occur when a large solid 28 has a large reverse (non-printed area) 30 within its boundaries. The leftover ink, as the form rollers pass over the reverse 30, is deposited further on, resulting in a darker ghost pattern 32.
The ghosting phenomenon disturbs not only the offset printing process. Letterpress printers also suffer from this problem, but to a lesser degree. Numerous attempts have been made to reduce mechanical ghosting.
U.S. Pat. Nos. 4,223,603 and 4,621,574 both to Faddis et al., U.S. Pat. No. 4,397,236 to Greiner, U.S. Pat. No. 4,777,877 to Lemaster. U.S. Pat. No. 4,584,940 to Germann, et al. and U.S. Pat. No. 5,062,362 to Kemp disclose different improvements to the offset-press inking unit. The improvements introduced affect the ways the ink roller train is driven. The roller train typically consists of a large number of rollers, which are required to smoothen the ink film. The improvements also affect the amplitude and, frequency of the oscillating rollers or the number of form rollers.
The numerous methods described above are all mechanical solution, adding cost and complexity to the printing press, and requiring a high degree of calibration. These methods, embodied in the physical construction of the press, are applied to each print, regardless of the probability of a ghost appearing thereon.
Water plays a part in this process too. It is used in the process to separate the regions of image area from non-image area. It is known that when a plate is well desensitized and water kept to a minimum, there is less ghosting. Water, however, may be trapped on image free plate areas and if not properly evacuated cause ink emulsification, blind spots on the print and overall degradation of the printed image quality. FIG. 2 is an illustration of a plate imposition pattern with a significant image tree area of the plate or with a significant area of plate filled with text only. In order to collect excessive ink from these low ink-consumption areas and reduce ink oxidation, solid ink take-off bars 36 are typically placed by the printer close to this area on the plate. Solid ink take-off bars 36, however, make this area prone to water-on-plate trapping since they xe2x80x98blockxe2x80x99 the water outlet.
Defining the amount of water required to print a particular image is more complicated than that of the amount of ink. Part of the water evaporates in the delivery train and part is evaporated from the paper before the actual printing occurs.
Sophisticated color offset presses have a water presetting system similar to the one used for ink setting, although there are no computerized programs available to preset the water or solution amount on the press.
Water balance for metallic and solid colors is essentially different from that usually selected for process color printing. These typically high ink-coverage areas have a higher water film thickness on the fountain roller than lower density process color prints.
Press operators have worked out a set of rules of thumb (basic-known in the trade rules or principles) to cope with the above problems. These include signature layouts that have an appropriate distribution of solids, halftones and type that may positively affect ghosting. When possible, they place solid areas near the gripper edge 34 to even out the plate""s ink demand.
A slight rotation (angling) of the plate or a 180xc2x0 rotation of the plate is sometimes used, when a high likelihood of ghosting exists. This method comprises the actual rotation of the offset plate after it is mounted on the press and first printing impressions have been produced. Sometimes, additional ink take-off bars are placed un the form to help get rid of excess ink. All of these methods are applied after a problem has been discovered during printing, are time consuming, and require a very high degree of accuracy and press operator skills.
FIG. 3A is an illustration of a plate imposition pattern 190, which is prone to mechanical ghosting. The plate 190 consists of 8 imposed pages 192, a control strip 194 and an optional ghosting prevention bar 196. The ghosting pattern produced by such a plate is typically discovered by a press operator after a number of impressions have been made. One of the well-known remedies for this problem is manual angling of the plate on the press, in a small angle Alpha of typically 1 to 1.5 degrees. The screening angles at the CMYK separations (also called xe2x80x98screen quartetxe2x80x99) are generally selected in such a way that they form the recommended minimum-moirxc3xa9 pattern by arranging the angles in 15, /5, 0 and 45 degrees, respectively, relative to one of the paper edges. A screen structure 198 and the 0-degrees angle created by the yellow separation are shown in an exaggerated form on FIG. 3A. Some staircase visual effects may appear on the edges of the image if the orientation of the xe2x80x98screen quartetxe2x80x99 with respect to the paper edge is not maintained.
FIG. 3B is an illustration of the offset plate 190 of FIG. 3A, manually angled on the press. When the plate is manually angled on the press, in an angle Alpha, the screen orientation with respect to the paper edge is kept constant. The paper is then cut according to the folding/cutting marks orientation, typically (except in packaging) parallel to the imaged page edges. This manual angling maintains the relative orientation of the xe2x80x98screen quartetxe2x80x99 with respect to the paper edge and avoids appearance of staircase effects on the edges of the image.
FIG. 3C is an illustration of a prior art electronically angled image of the offset plate 190, with an imposition pattern prone to mechanical ghosting. In this case, the plate 190 does not change its orientation on the press, but rather, each page 192 is rotated in a small, typically 1 to 1.5 degrees angle Alpha. The screening angles of the CMYK separations remain, however, oriented with respect to the imaging device axis. Once again, the screen structure 198 and the angle Alpha created by the yellow separation are shown in an exaggerated form in FIG. 3C. Some staircase visual effects will inevitably appear on the edges of the image, if the relative orientation of the xe2x80x98screen quartetxe2x80x99 with respect to the paper edge is not maintained. The rotation of the separate pages 192 also changes the alignment of the printer spread (marked in broken lines), creating a difference 200 between their respective bottom/top boundaries positions. If the control strip 194 is not rotated and its dimensions are not adjusted, it will not cover the whole printing image area, leaving gaps 202 at both ends of the control strip 194.
FIG. 3D is an illustration of the plate 190 of FIG. 3C, with control strip 194 electronically rotated, according to the prior art. Rotation of such a long, typically 1-meter (40-inch) strip, even in a small one-degree angle, displaces the end of the strip a relatively large distance 204 of more than 17 mm. Such a large displacement may not leave enough space for inserting ghosting prevention bars and thus complicates mechanical ghosting prevention.
There are a number of disadvantages with state of the art imposition systems such as Brisqueimpose or Preps Pro, which is commercially available from ScenicSoft, Inc. of Everett, Wash. U.S.A. These systems disregard ghosting considerations, as well as other quality related parameters such as minimal dot size and screen structure, their compatibility to the paper grade, coating, surface roughness, and fountain solution of the particular printing press. These also affect the quality of the printed image.
The present invention is directed to a prepress and press system including the means and methods for predicting press related problems and preventing them digitally.
According to a first aspect of the present invention, a computerized prepress and press system is presented, the system including imposition means and ghosting prediction means. The imposition means receive from a digital storage means at least one digital representation of at least one page and arrange the at least one digital representation in accordance with a desired plate layout, thereby defining a digital plate image. The ghosting prediction means are coupled to the imposition means and are operable to predict, based on one of the digital plate image data and a relatively low resolution version of the digital plate image data, where ghosting will occur on the printed image.
According to another aspect of the present invention, there is also presented ghosting prevention means coupled to the ghosting prediction means and to the imposition means. The ghosting prevention means are operable to calculate positions and color values for ghosting prevention bars, using the ghosting prediction.
According to yet another aspect of the present invention, there is also presented means for automatically inserting ghosting prevention bars into the digital plate image.
According to another aspect of the present invention, there is also presented water trapping prediction means coupled to the ghosting prediction means and to the imposition means. The water trapping prediction means are operable to predict, based on the digital plate image data, where water-trapping will occur on the plate.
According to yet another aspect of the present invention, there is also presented water trapping compensation means, coupled to the water trapping prediction means, operable to calculate positions and sizes for water-trapping prevention bars, using the water trapping prediction.
According to yet another aspect of the present invention, there is also presented ink trap correction means coupled to the imposition means, operable to correct zonal ink key setting for wet ink trapping.
According to yet another aspect of the present invention, there is also presented means for transferring said corrected zonal ink key setting to a press set-up means of the press.
According to yet another aspect of the present invention, there is also presented means for calculating automatic water setting.
According to yet another aspect of the present invention, the means for calculating automatic water setting include means for calculating automatic water acting for spot colors.
According to yet another aspect of the present invention, the means for calculating automatic water setting include means for calculating automatic water setting for metal colors.
According to yet another aspect of the present invention, there is also presented means for transferring the automatic water setting to a press set-up means of the press.
In another aspect, the present invention presents a method for creating a digital image of a printing plate, including the step of creating a first digital image of the printing plate, using digital representation of at least one page and given coordinates thereof on the printing plate. The method also includes the stop of using one of the first digital image of the printing plate and a relatively low resolution version of the first digital image of the printing plate to predict where ghosting will occur on the printed image. The method also includes the step of using the first digital image of the printing plate and the ghosting prediction to create a second digital image of the printing plate, the second digital image including a ghosting prevention bar.
In yet another aspect of the present invention, the ghosting prevention includes the step of inserting the ghosting prevention bar into the first digital image.
In yet another aspect of the present invention, the ghosting prevention bar is divided into segments.
In yet another aspect of the present invention, the ghosting prevention includes the step of rotating the at least one page of the first digital image by a rotation angle and rotating the screen pattern of the at least one page by the same rotation angle.
In yet another aspect of the present invention, the ghosting prevention includes the step of rotating at least one printer-spread of the first digital image and rotating the screen pattern of the at least one printer-spread by the same rotation angle.
In yet another aspect, the method for creating a digital image of a printing plate additionally includes the step of dividing the color control bar of the digital image into segments.
In yet another aspect the method for creating a digital image of a printing plate additionally includes the steps of using a digital image of the printing plate to predict where water trapping will occur on the printing plate, and using the digital image of the printing plate and the prediction of water trapping to create a new digital image of the printing plate, the new digital image including digital water take-off bars.